1. Field of the Invention
This invention relates to an array of antennas used to transmit electromagnetic energy to a distant target, for example to an aircraft to provide its motive power, and more particularly to means for controlling the relative phases of the energy radiated by the various antennas so as to maximise continually the energy received at the target.
2. Discussion of the Prior Art
In a SHARP (Stationary High Altitude Relay Platform) system, a large ground antenna subsystem transmits microwave power to the airborne vehicle where the power is captured by a "rectenna" (a small array of antennas, each with its own rectifier) and used to propel the aircraft and to power the payload. If the ground subsystem consists of several steerable antennas, not only must each antenna be pointed at the target, but for maximum power transmission efficiency, the powers from the various antennas must arrive at the aircraft in phase. On the other hand, if a classical "phased array" is used, with a large number of fixed radiators, in order to form a beam at all and then to steer and focus it, the phases of the radiators must be adjusted either individually or at least in small groups so that the signals from the radiators all arrive at the target substantially in phase with each other.
In the description which follows, an antenna is an individual radiator or a group of radiators, in either case controlled by a single phase shifter.